In integrated circuit technology, the general function of a voltage generator or regulator is to provide a reference potential for use by individual or combinations of circuit components. For many applications, a regulated voltage source for providing a stable reference voltage level is an essential element. However, it is well known that circuit devices, particularly semiconductor components, have operating characteristics which are dependent upon both inherent physico-chemical aspects of the materials used in the construction of the devices and operating conditions under which they are required to perform their designed functions.
Metal-oxide semiconductor field effect transistors (MOSFET) are a natural choice for digital applications because of their ON, OFF method of operation. The mode at any particular point in time is controlled by a threshold voltage; i.e., the minimum applied gate-to-source voltage required to electrically invert the semiconductor layer under an insulated gate region, creating a conduction channel or, more simplistically, turning the MOSFET ON. The threshold voltage characteristic of any particular MOSFET is affected by both said commonly known, inherent characteristics and by the operating temperature which affects both these characteristics and the effective mobility of the carriers within the semiconductor materials of the device.
There is an important need, therefore, for reference voltage generators which exhibit long-term stability and insensitivity to fluctuations in the device's operating temperature and threshold voltage changes. For example, a key element to achieving a good endurance electrically erasable programmable read only memory (EEPROM) integrated circuit at an acceptable yield is a temperature insensitive voltage reference source, which controls the WRITE voltages.
A depletion mode MOSFET has an additional ion implantation directly into the channel region; said implant removes the requirement for a threshold voltage since these devices will have a channel and will conduct a current with a gatesource voltage of zero volts. Hence, a separate gate bias potential supply is eliminated.
A technique of combining depletion transistors with intrinsic transistors in an attempt to overcome these problems was proposed by R. A. Blauschild et al: New NMOS Temperature-Stable Voltage Reference, IEEE 2. A Solid-State Circuits, Vol. SC-13, No. 6, Dec. 1978, pp. 767-774. A stable voltage reference was obtained by a circuit generating a voltage proportional to the difference between enhancement mode transistor and depletion mode transistor threshold voltages. Basically, as temperature increases, both type transistor thresholds decrease with a similar slope of 2.3 mV/degree centigrade. This circuit is shown in FIG. 1 (prior art). However, the nature of the circuit makes implementation best suited for double power supply applications. Additionally, this complex technique requires a relatively large geometry section of the integrated circuit into which it is incorporated.